pH‐Gated Switch of LCST‐UCST Phase Transition of Hydrogels

Abstract Temperature‐responsive hydrogels including lower critical solution temperature (LCST)‐ and upper critical solution temperature (UCST)‐type hydrogels are attractive in various fields. However, the swift switch between LCST and UCST stimuli‐responsive behaviors remains intriguing and challenging. Here is reported a kind of hydrogel with pH‐gated LCST and UCST response behaviors. This is achieved using the hydrogen bonding between carboxylic acid groups of poly(acrylic acid‐ co ‐acrylamide) and hydroxyl groups of hydroxypropyl cellulose (HPC). The poly(acrylic acid‐ co ‐acrylamide)‐HPC (PACA‐HPC) hydrogels exhibit pH‐gated reversible LCST‐UCST phase transition behavior. When the transparent PACA‐HPC hydrogel is placed in an acid bath, the COO − groups get protonated, rapidly forming hydrogen bonds with HPC to render a light‐scattering state making the hydrogel opaque. Furthermore, the opaque hydrogel exhibits UCST phase transition behavior at 20–45 °C. When the opaque PCAC‐HPC hydrogel is placed in an alkaline environment, hydrogen‐bonded complexes gradually dissociate as the COOH groups are deprotonated to form a homogeneous transparent state. The transparent hydrogel exhibits LCST phase transition behavior at 20–45° C. Therefore, is shown the hydrogen bonding strategy to fabricate hydrogels with tunable LCST and UCST responses. With this pH‐gated hydrogel with switchable LCST/UCST responsive behaviors, are demonstrated its applications in smart windows and information encryption.